The present invention relates to storing data to sequential storage media, and more particularly, to improved data storage utilizing rewrites having dead-track detection.
Currently-used linear tape drives which are used to store data sequentially apply product codes for byte-oriented error-correction coding (ECC) to the data prior to storing the data to the tape. These product codes contain two Reed-Solomon (RS) component codes consisting of a C1 row code and a C2 column code. Relatively long (about 1 kB) longitudinal interleaved error correction codewords, also known as codeword interleaves (CWI), are written on tracks of the magnetic medium (e.g., magnetic tape tracks). In current tape drive architectures, CWIs consist of four byte-interleaved RS codewords. During read-while-write, CWIs in a data set (DS) that includes more than a threshold number of errors are rewritten after the writing of the DS that has too many errors therein has been completed. Two CWI sets correspond to a codeword object (CO) set, which represents the minimum amount of data that can be written, or rewritten, on tape using current methodologies. This operating point is selected such that at the beginning of a tape drive's lifetime, the average number of rewritten CWI sets per DS is limited to about two rewritten CWI sets per DS, which corresponds to a 1% rewrite rate. The rewrite rate of 1% is currently reached when a byte error rate at the C1 decoder input is around 1×10−4.
In current tape drives in the presence of one or more dead tracks, CWIs are rewritten on these dead tracks, which increases the rewrite rate above the desired threshold because of the delay that occurs between the rewriting of a CWI on a dead track and the subsequent reading of that CWI as rewritten on a dead track during read-while-write. This latency is due to a distance between the read element and write element on the head, about 800 microns, and the latency associated with decoding a CWI, about the same as the time needed to read four consecutive CWIs on a track. In other words, a CWI rewritten on a dead track results in an increase of the rewrite rate by at least 2% at the beginning of a tape drive's lifetime.